People worry about weeds in their lawn in the summer, but the best time to inspect your lawn for weeds is in the winter. Grass tends to go dormant in the the winter and turn yellow. Weeds keep their green color and stick out for you to easily spot. Some winter weeds to look out for are white clover, henbit, common chickweed, and annua poa. You should use a selective herbicide labeled for these weeds to take them out easily during the winter. These selective herbicides usually contain atrazine or 2,4-D, but not both. You can use a small amount of product and get rid of the weeds without treating your whole lawn.
You will not see certain summer weeds like crabgrass since they go away for winter since it is an annual weed. If you know you had a problem with crabgrass in the summer, the winter is a great time to apply a pre-emergent herbicide to prevent the seeds left from sprouting during the warm season. You can use a pre-emergent to treat many of the annual weeds you see in the summer that go away in winter. If you remember where they grew, you can treat those areas for better control.
Also, do not add any weed and feed for your lawn. Nitrogen promotes growth, and your grass should not be growing.